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Pcb's Environmental Impact, Biochemical & Toxic

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Pcb's Environmental Impact, Biochemical & Toxic ^. 012 pru vrui npiMATir.p RIPHFXVI <; (prRO- ENVIRONMENTAL IMPACT, BIOCHEMICAL AND TOXIC RESPONSES AND IMPLICATIONS FOR RISK ASSESSMENT 5. Safe Department of Veterinary Physiology and Pharmacology Texas A&M University College Station, Texas 77843-4466 ABSTRACT Commercial polychlorinated biphenyls (PCBs) and environmental extracts contain complex mixtures of isomers and congeners which can be unequivocally identified and quantitated. PCBs elicit a spectrum of biochemical and toxic responses in humans and laboratory animals and many of these effects resemble those caused by 2.3,7,8- tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons which act through the aryl hydrocarbon (Ah) receptor signal transduction pathway. Structure- activity relationships developed for PCB congeners and metabolites have demonstrated that several structural classes of compounds exhibit diverse biochemical and toxic responses. Structure-toxicity studies suggest that the coplanar PCBs, namely 3.3 ',4,4'- tetrachlorobiphenyl (tetraCB), 3,3',4,4'-pentaCB and 3,3',4,4',5,5'-hexaCB and their monoortho analogs are Ah receptor agonists and contribute significantly to the toxicity of the PCB mixtures. Previous studies with TCDD and structurally related compounds have utilized a toxic equivalency factor (TEF) approach for the hazard and risk assessment of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) congeners in which the TCDD or toxic equivalents (TEQ) of a mixture is related to the TEQ = £ ( [ PCDF, x TEF, ]„ ) + £ ( [ PCDD, x TEFt ]„ ) TEFs and concentrations of the individual (i) congeners (note: n = the number of congeners). Based on the results of quantitative structure-activity data, the following TEFs are recommended for the coplanar and monoortho coplanar PCBs: 3,3',4,4',5-pentaCB, 0.1; 3,3',4,4',5,5'-hexaCB, 0.05; 3,3',4,4'-tetraCB, 0.02; 2,3,3', 4,4 '-pentaCB, 0.001; 2,3',4,4',5- pentaCB, 0.0001; 2,3,3',4,4',5-hexaCB, 0.0003; 2,3,3',4,4',5'-hexaCB, 0.0003; 2',3,4,4',5- pentaCB. 0.00005; and 2.3.4,4'.5-pentaCB. 0.0002. Application of the TEF approach for the risk assessment of PCBs must be used with considerable caution. Analysis of the results of laboratory animal and wildlife studies suggests that the predictive value of TEQs for PCBs may be both species- and response-dependent since both additive.and non-additive (antagonistic) interactions have been observed with PCB mixtures. In the latter case, the TEF approach would significantly overestimate the toxicity of a PCB mixture. Analysis of the rodent carcinogenicity data for Aroclor 1260 using the TEF approach suggests that this response is primarily Ah receptor-independent. Thus, risk assessment of PCB mixtures which uses cancer as an endpoint cannot utilize a TEF approach and requires more quantitative information on the individual congeners which contribute to the tumor promoter activity of PCB mixtures. I. INTRODUCTION Polychlorinated biphenyls (PCBs) are members of the halogenated aromatic group of environmental pollutants which have been identified worldwide in diverse environmental matrices. PCBs are produced by the chlormation of biphenyl and the resulting products are marketed according to their % chlorine content (by weight)45-142-232-455. For example, Aroclor 1221, 1232. 1242, 1248, 1254 and 1260 are commercial PCBs which were formerly produced by the Monsanto Chemical Company and contain 21, 32, 42, 48, 54 and 60% chlorine (by weight). The last two digits in the numerical designation for the different Aroclors denotes the % chlorine content. Similar commercial PCB mixtures have been produced by other manufacturers and these include the Clophens (Bayer, Germany), Phenoclors and Pyralenes (Prodelec, France), Fenclors (Caffaro, Italy) and Kanechlors (Kanegafuchi, Japan). Commercial PCBs have also been manufactured in several other countries including the former U.S.S.R. and Czechoslovakia. The commercial PCBs exhibit a broad range of physicochemical properties which are dependent, in part, on their degree of chlorination and these properties contribute to the diverse applications of PCBs in numerous products. For example, PCBs have been used as organic diluents, pesticide extenders, adhesives, dedusting agents, cutting oils, flame retardants, heat transfer fluids, dielectric fluids for transformers and capacitors, hydraulic lubricants, sealants, and in carbonless copy paper. Some of the uses of PCBs have resulted in the direct introduction of PCBs into the environment; however, a significant portion of the environmental burden of these compounds has resulted from careless disposal practices, accidents, leakage from various industrial facilities, and from chemical waste disposal sites. The total amount of PCBs produced worldwide and the proportion which is present in the environment is unknown; however, it has been estimated 5 that 1.5 million metric tons have been produced worldwide M2. The chemical properties which are primarily responsible for many of the industrial applications of PCBs, namely their chemical stability and miscibility with organic compounds (i.e. lipophilicity), are also the same properties which have contributed to their environmental problems. Once introduced into the environment, the relatively stable PCBs resist environmental breakdown and undergo cycling and transport within the various components of the global ecosystem. Moreover, due to the lipophilicity of PCBs, these compounds preferentially bioaccumulate and biomagnify in higher trophic levels of the food chain 2M•M9-459•4*°. The environmental problems and the difficulties encountered in the hazard and risk assessment of PCBs have also arisen because of another chemical property of these compounds, namely the relatively non-specific chlorination of biphenyl232. This lack of chlorination site specificity means that the commercial PCB products and environmental PCB residues are complex mixtures of isomers and congeners. Thus, the impacts of PCBs on the environment and biota are due to the individual components of these mixtures, their additive and/or non-additive (synergistic or antagonist) interactions with themselves and other chemical classes of pollutants. Therefore, the development of scientifically-based regulations for the risk management of PCBs which would protect against adverse environmental and human health effects would require analytical and lexicological data on the individual PCB congeners present in any PCB mixture and information regarding their interactive effects. There are significant challenges associated with a congener-specific approach for the analysis and risk assessment of PCBs and these studies are currently ongoing in several laboratories and regulatory agencies. This review will focus on some of the more recent studies which have added to our understanding of PCBs and will also 6 discuss problems associated with PCB toxicology and risk assessment which are ongoing and have not yet been resolved. II. PCBs: ENVIRONMENTAL IMPACT The development of improved techniques for PCB analysis has played a pivotal role in understanding the environmental fate and potential adverse human health and environmental impacts of PCBs. In the late 1960s, Soren Jensen first detected PCBs in environmental samples as a series of complex peaks which were observed in a gas chromatographic screening of environmental samples for DDT and related compounds 26. Subsequent studies in several laboratories have identified PCBs in almost every component of the global ecosystem including air, water, sediments, fish, wildlife and human tissues 3I H. 4M5. 77. ,70. 20,. 2,9. 235. 260. 262. 263. 307. 372.43,. 460, 522. 529-534. 573. 583 j^ Q{ ^ ^ ^^ ^j^ low resolution packed column gas chromatographic separation of the PCB mixtures in which concentrations were determined by matching specific peak patterns and their intensities with the corresponding peaks in commercial PCBs or combinations of different commercial mixtures which were used as standards 57S. The criteria for the selection of commercial PCB standards were variable; however, in many cases, the choice was due to the similarities between the chromatographic peak patterns observed for the standard mixture and the PCBs in the analyte. High resolution analysis for PCB mixtures was first reported by Sissons and Welti501 and there has been continued improvements in both the resolution capabilities of capillary columns and detection methods 8 '• 47' 263- 369-371 416-457- 48°-5<n. However, the unambiguous identification of the 209 possible PCB congeners required the synthesis of all these compounds and their subsequent use as analytical standards. This synthesis was 7 reported in 1984 371 and subsequent studies have identified and quantitated all the PCB congeners present in several different Aroclor and Clophen mixtures JS0. A total of 132 different individual PCBs were identified in these mixtures at concentrations > 0.05% (w/w) and the congener composition of each PCB mixture was dependent on their chlorine composition 48°. Inspection of the analytical data shows that some congeners occur in only one of the PCB mixtures whereas others are detected in all of the mixtures. PCBs have been identified as residues from extracts of diverse environmental samples. In all cases, the PCBs are present as complex mixtures of isomers and congeners and, until recently, most routine analytical surveys reported "total PCB" levels using the peak matching technique with commercial Aroclors as standards 575. The PCB levels
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